1. Field of the Invention
The present invention relates to the structure of an optical cable for holding optical fiber ribbons.
2. Description of the Related Art
Optical cables are generally made through the steps of twining one-groove spacers around a central member, each one-groove spacer having a single groove which is substantially square in cross section and linear lengthwise, and holding an optical fiber ribbon or a stack of a plurality of optical fiber ribbons into the groove, or holding an optical fiber ribbon or a stack of a plurality of optical fiber ribbons into the respective grooves of a grooved spacer.
The optical cable disclosed in Unexamined Japanese Patent Publication No. Hei. 4-182608 has one-groove spacers wherein the inner width and the height of the side walls of the groove of each one-groove spacer are set greater than the diagonal length of a stack of a plurality of optical fiber ribbons so that the optical fiber ribbons are made freely movable in the groove. This structure is intended to prevent the optical cable from being damaged by bending stress acting from the outside during the manufacture, transport or laying of optical cables.
In such a conventional optical cable, however, the optical fibers of the optical fiber ribbons which are placed at a same side are kept in contact with the groove as illustrated in FIG. 5A. In FIG. 5A, reference numeral 3 denotes a one-groove spacer; and 4, optical fiber ribbons. When the bending stress is applied to the optical cable, the optical fiber ribbons 4 are pressed against the one side wall of the one-groove spacer 3 and the stress is concentrated on the portion indicated by the mark x. Consequently, the specific optical fibers which are in contact with the grooves are always subjected to the stress.
This is also the case with an optical cable having SZ-twisted one-groove spacers, that is, a plurality of one-groove spacers are twisted around a central member while reversing their direction at a given pitch length or SZ-shaped grooves of the grooved spacers, that is, grooves are formed so as to reverse their direction at predetermined pitch length. FIG. 7 is a perspective view of a grooved spacer having SZ-shaped grooves. In FIG. 7, reference numerical 111 denotes a grooved spacer; 112, an anti-tensile element; 113, an SZ-shaped grooves; and 114, an optical fiber ribbon. While not shown in FIG. 7, the grooved spacer 111 has a plurality of grooves 113. Symbols S1 and S2 indicate reversing portions, and symbol S0 a conversion portion. The grooves 113 are formed so as to reverse their direction at predetermined pitch length, for example the predetermined pitch length being a length between the reversing portions S1 and S2. In the case of SZ twisting, only a limited number of optical fibers are brought into contact with the wall of the groove. Further, a greater stress is applied to the reversing portion S1, S2 where the twisting direction is reversed, so that specific optical fibers receive that greater stress. Therefore, the edgewise pressure applied to the specific optical fibers tends to become greater in the conventional optical cables, thereby results in an increase of transmission loss.
Even in the optical cable disclosed in the above-mentioned Unexamined Japanese Patent Publication No. Hei. 4-182608 wherein the optical fiber ribbons are allowed to freely move in the grooves, the stack of a plurality of optical fiber ribbons are as shown in FIG. 2 pressed against the side wall in one direction, which results in subjecting the optical fibers in contact with the side wall to the edgewise pressure, thus causing a micro bend loss. Therefore, only making the inner width and the height of the side walls of the groove greater than the diagonal length of the stack still remain insufficient to reduce the loss.
Accordingly, an object of the present invention is to provide an optical cable wherein specific optical fibers are set free from an increase in transmission loss.
The above-mentioned object can be achieved by an optical cable having at least one groove which is spirally provided, the each groove being substantially square in cross section and holding an optical fiber ribbon or a stack of a plurality of optical fiber ribbons within the groove. In the cable, an inner width and a height of side walls of the each groove are set greater than a width of the optical fiber ribbon or a diagonal length of the stack which is held into the groove and the optical fiber ribbon or the stack held into the groove is twisted lengthwise in one direction.
In accordance with a first aspect of the present invention, in the above-mentioned optical cable, it is preferable that the optical cable further comprises a central member and at least one spacer having the at least one groove, wherein the spacer is twisted around the central member spirally. For example, the spacer is one-groove spacer having the one groove being linear lengthwise.
In the above-mentioned structure, it is advantageous that the one-groove spacer is twisted in one direction around the central member and a twisting pitch length of the optical fiber ribbons is equal to or less than a twisting pitch length of the one-groove spacer.
It is also advantageous that the one-groove spacer is twisted around the central member while reversing its direction at a given pitch length. In such a optical cable, it is preferable that a twisting pitch length of the optical fiber ribbons is equal to or shorter than a pitch length of the one-groove spacer, the pitch length of the one-groove spacer being twice as large as the given pitch length, and the pitch length of the one-groove spacer is not integer times as large as the twisting pitch length of the optical fiber ribbons.
Further, in the above-mentioned structure, it is advantageous that the central member is a grooved spacer having at least one groove on its surface, each groove being spiral lengthwise and substantially square in cross section and holding an optical fiber ribbon or a stack of a plurality of optical fiber ribbons within the groove. In such a optical cable, it is preferable that an inner width and a height of side walls of the each groove of the grooved spacer are set greater than a width of the optical fiber ribbon or a diagonal length of the stack which is held into the groove of the grooved spacer and the optical fiber ribbon or the stack held into the groove of the grooved spacer is twisted lengthwise in one direction.
It is also advantageous that the optical cable further comprises one-groove spacers spirally being twisted around the one-groove spacers which are twisting around the central member.
In accordance with a second aspect of the present invention, in the above-mentioned optical cable, it is preferable that the optical cable further comprises a central member, wherein the central member is a grooved spacer having the at least one groove on its surface, each groove being spiral lengthwise.
In the above-mentioned structure, it is advantageous that the groove is a spiral groove being spirally formed in one direction on the surface of the grooved spacer and a twisting pitch length of the optical fiber ribbons is equal to or less than a spiral pitch length of the groove of the grooved spacer.
Further, it is advantageous that the groove is an SZ-shaped groove being spirally formed so as to reverse their at predetermined pitch length. In such an optical cable, it is preferable that a twisting pitch length of the optical fiber ribbons is equal to or shorter than a pitch length of the grooved spacer, the pitch length of the grooved spacer being twice as large as the predetermined pitch length, i.e. the length between reversing portions S1 and S3 as depicted in FIG. 7, and the pitch length of the groover spacer is not integer times as large as the twisting pitch length of the optical fiber ribbons.